System and method for tape drive control

ABSTRACT

A system and method for damping the motion of the head of a tape drive is disclosed. The method includes measuring the position of the head over time with a position sensor; damping the motion of the head based on the measured position of the head over time; and tuning the damping by varying a parameter of the position sensor.

FIELD OF THE INVENTION

The present invention relates to tape drives. More particularly, thepresent invention relates to damping the movement of the read-write headof a tape drive.

BACKGROUND OF THE INVENTION

High-density recording on multiple tracks of a magnetic tape is known.In certain arrangements, parallel tracks extend along a longitudinaldirection of the magnetic tape. The magnetic tape is moved transverselyacross a read/write head and data is recorded or read. During recordingor playback, the head needs to stay in a fixed lateral position relativeto the tape as the tape moves in a longitudinal direction.

Some existing tape storage systems contain a mechanism that allows theread/write head to be located at the track's centerline at the beginningof the read/write process. Once the read/write process begins, there isno correction if an offset should arise between the head and thecenterline of the track.

To increase storage capacities to meet increased demands, track density,which is the number of tracks per distance (e.g., inches), should beincreased. As track density increases, the track pitch and widthdecrease. For proper read/write operation, the magnetic head must stayat, or very near, the centerline of the track, and the need forprecision increases as track density increases.

A conventional tape drive system is shown in FIG. 1. This systemcomprises a tape drive 12 connected to a host computer 10 by a cable 16,and an associated tape cartridge 14. The tape drive 12 includes areceiving slot 22 into which the tape cartridge 14 is inserted. The tapecartridge 14 comprises a housing 18 containing a length of magnetic tape20. The tape drive 12 is preferably compatible with the associated hostcomputer, and can assume any one of a variety of cartridge or cassettelinear formats.

A typical configuration of the tape drive 12 is shown in FIG. 2. Thetape drive 12 in FIG. 2 comprises a deck 24 including movable parts, anda control card 26 including various circuits and buses. The deck 24includes a head assembly 28 which contacts the tape 20 of the tapecartridge inserted into the tape drive 12 to read and write data andread a servo pattern, and motors 34 and 36 for respectively rotating asupply reel 30 and a take-up reel 32. For a tape cartridge 14 of a dualreel type, both of the reels 30 and 32 are included in the tapecartridge 14. For a tape cartridge 14 of a single reel type, however,only the supply reel 30 is included in the tape cartridge 14 while thetake-up reel 32 is provided in the tape drive 12. Although not shown inFIG. 2, the deck 24 additionally includes a mechanism for moving thehead assembly 28 across the width of the tape 20, a mechanism forholding the inserted tape cartridge, and a mechanism for ejecting theinserted tape cartridge.

The control card 26 includes a microprocessor (MPU) 38 for the overallcontrol of the tape drive 12; a memory 42, a servo control unit 44, adata flow unit 46 and an interface control unit 48 all of which areconnected to the MPU 38 via an internal bus 40; a motor control unit 50and a head control unit 52 which are connected to the servo control unit44; and a data channel unit 54 which is connected to the data flow unit46. While the memory 42 is shown as a single hardware component in FIG.2, it is actually preferably constituted by a read only memory (ROM)storing a program to be executed by the MPU 38, and a working randomaccess memory (RAM). The servo control unit 44 manages speed control forthe motors 34 and 36 and position control for the head assembly 28 bytransmitting the respective control signals to the motor control unit 50and the head control unit 52. The motor and head control units 50 and 52respond to these control signals by physically driving the motors 34, 36and positioning the head assembly 28, respectively.

The head assembly 28 includes servo heads which read data from servotracks or bands on the tape 20. Control card 26 utilizes data from theservo tracks to generate a position error signal (PES), and the PES isused by the servo control unit 44 to cause the head control unit 52 toposition the head assembly 28. In some conventional designs the headassembly 28 includes a voice coil motor (VCM) 56 which receiveselectrical signals from the head control unit 52 and positions the headassembly 28 according to the received signals.

The data flow unit 46 compresses data to be written on the tape 20,decompresses data read from the tape 20 and corrects errors, and isconnected not only to the data channel unit 54 but also to the interfacecontrol unit 48. The interface control unit 48 is provided tocommunicate data to/from the host computer 10 via the cable 16. The datachannel unit 54 is essentially a data modulating and demodulatingcircuit. That is, when data is written to the tape 20, it performsdigital-analog conversion and modulation for data received from the dataflow unit 46, and when data is read from the tape 20, it performsanalog-digital conversion and demodulation for data read by the headassembly 28.

In some types of tape drives the head control unit 52 and head assemblycan be considered to be essentially a second order type spring-massactuator system. Actuator systems of this type can have a fundamentalresonance that is normally within the loop bandwidth. One of the commonfeatures of a second order type spring-mass actuator system is the lackof damping unless damping is designed in as part of the mechanicalassembly. Damping provides a controlled step response to the actuator.Too much damping will make the system sluggish while the lack of dampingwill cause it to ring at the fundamental frequency.

In some applications, damping is achieved by mechanical means, such asthe use of grease, ferro fluids, etc. A system of this type is taught inU.S. Pat. No. 5,739,984. In other applications, either the PES or backEMF signal is used to estimate the velocity state variable for the headassembly 28 to be damped, and this information is fed back to the servocontrol unit 44. U.S. Pat. No. 6,359,748 teaches such use of a back EMFsignal. However, these two methods have certain disadvantages. Forexample, PES is continuous only in defined zones and also it has therelative position information between the head and the tape making theestimation process rather difficult and limited to the availability ofPES. In some situations the servo heads are very near the edges of aservo band. Shock and vibration disturbances as well as large lateraltape motion due to staggered wrap tape layers may cause the servo headto move outside the servo band thereby causing loss of the PES. The backEMF signal is difficult to use since its value is normally dependent onthe electrical and physical parameters of the actuator, inductance, coilresistance, sense resistance, and magnetic characteristics, making thetask to estimate the actuator velocity as a function of back EMF a verydifficult and potentially inaccurate. Moreover the back EMF signal isnot tunable.

For systems which have predefined sections of tape where the feedbacksignal is located, such as the Linear Tape Open (LTO) servo feedbackmethod, discontinuous type feedback signals can present a problem.Discontinuous type servo signals occur for e.g. the following reasons:If the tape suddenly moves up or down the head can leave the servo band.While the head is tracking very near the edges of the servo bandexternal shock or vibration can push the head off the servo band.

The use of a VCM with little or no damping in an environment thatexhibits discontinuous type feedback can cause the head to oscillate atits natural frequency. With the head oscillating, the reacquisition oftracking normally takes time which results in reduction of systemperformance.

Accordingly, it is an object of the present invention to provide asystem and method to reduce head oscillation. It is another object ofthe invention to reduce head oscillation by providing a tunable dampingsystem including an optical sensor to sense the position of the head.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent invention and, together with the detailed description, serve toexplain the principles and implementations of the invention.

In the drawings:

FIG. 1 is a view of a conventional tape drive system.

FIG. 2 is a block diagram showing an exemplary arrangement of a controlcard and tape drive according to the prior art.

FIG. 3 is an exploded view of a portion of an optical position sensoraccording to a preferred embodiment of the present invention.

FIG. 4 is a view of the components shown in FIG. 3 with the componentsassembled.

FIG. 5 is a view of the optical position sensor according to a preferredembodiment of the present invention as connected to the read-write headand the VCM.

FIG. 6 is a cross sectional view of the optical position sensor, theread-write head and the VCM.

FIG. 6A is a partially exploded, isometric view of the optical positionsensor, the read-write head and the VCM.

FIG. 7 is a block diagram of a control system according to a preferredembodiment of the present invention.

FIG. 8 is another block diagram of a control system according to apreferred embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention are described herein in the contextof a system and method for tape drive control. Those of ordinary skillin the art will realize that the following detailed description of thepresent invention is illustrative only and is not intended to be in anyway limiting. Other embodiments of the present invention will readilysuggest themselves to such skilled persons having the benefit of thisdisclosure. Reference will now be made in detail to implementations ofthe present invention as illustrated in the accompanying drawings. Thesame reference indicators will be used throughout the drawings and thefollowing detailed description to refer to the same or like parts.

In the interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application- and business-related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

In accordance with the present invention, the components, process steps,and/or data structures may be implemented using various types ofoperating systems, computing platforms, computer programs, and/orgeneral purpose machines. In addition, those of ordinary skill in theart will recognize that devices of a less general purpose nature, suchas hardwired devices, field programmable gate arrays (FPGAs),application specific integrated circuits (ASICs), or the like, may alsobe used without departing from the scope and spirit of the inventiveconcepts disclosed herein.

Turning now to FIGS. 3–6A there is shown an optical position sensor 59which includes optical emitter assembly 60, which includes a lightemitting diode, LED 62, and light sensor assembly 64. The emitterassembly 60 and light sensor assembly 64 are mounted in housing body 66and housing cap 68. The optical position sensor 59 is assembled withread-write head assembly 72 and VCM 102. An adjustable screw 74 isattached in threaded engagement with a center spring clamp 78. FIG. 6shows the adjustable screw 74 located adjacent to the LED 62 so that thehead of the screw 74 partially interrupts the light beam from the LED 62to the light sensor assembly 64. The adjustable screw can be rotated tomove up and down to a selected position as indicated by arrow 76 for thepurpose of calibration.

As best shown in FIG. 6A, the VCM 102 comprises a voice coil assembly 80which is substantially cylindrical, and the voice coil assembly 80 islocated in a cylindrical bore formed in a voice coil housing 82. A topspring element 84 is connected to the top of the voice coil assembly 80and the voice coil housing 82 by center spring clamp 78, and the topspring element 84 provides a flexible connection between assembly 80 andhousing 84. A similar bottom spring element, not shown, is connected tothe bottom of the voice coil assembly 80 and the voice coil housing 82.The top and bottom spring elements provide a flexible connection andpermit limited vertical motion of the voice coil assembly 80 relative tothe voice coil housing 82. The read-write head assembly 72 is rigidlyconnected to the center spring clamp 78.

It should now be understood that the read-write head 72, the voice coilassembly 80, the center spring clamp 78 and adjustable screw 74 are allrigidly connected to each other so that they all move up and downtogether as a unit. On the other hand, optical position sensor 59 isisolated from those components so that optical position sensor 59 doesnot move with those components. Rather, the optical position sensor 59is rigidly connected to the voice coil housing 82, which, in turn isrigidly connected to the housing of the tape drive. Accordingly, as theread-write head assembly 72 moves up and down as indicated by arrow 86,the head of the adjustable screw 74 partially interrupts the beam oflight received by the light sensor assembly 64 from the LED 62, and theextent of light interruption is directly proportional to the relativevertical position of the optical position sensor 59 with respect to theread-write head assembly 72. The light sensor assembly 64 produceselectrical signals corresponding to the light it receives, and theelectrical signals are used by the control system discussed hereinafter.

Turning now to FIG. 7 there is shown a block diagram schematicallyillustrating a control system 98 according to a preferred embodiment ofthe present invention. The control system includes a summer 101, anamplifier 100, and a VCM 102, which is affixed to a head assembly 28.The gain of the amplifier 100 is Ka, and the transfer function of theVCM and head assembly is Gp. The control system 98 further includesfeedback loop 104 including the optical sensor 60 and a filter 106, andthe block 108 indicates that the gain C of the feedback loop can becontrolled. The filter 106 is AC coupled and has the transfer function:

$F = \frac{ks}{{\tau\; s} + 1}$

The variable k is the gain and T is the time constant. The filter 106can be a lead-lag filter which differentiates the input signal andapplies low pass filtering.

The control system 98 is connected so that the summer 101 receives asignal from the tracking loop and subtracts from the tracking loopsignal a signal from the feedback loop 104. The signal from the trackingloop represents the target position of the VCM and is generated by theservo control unit 44 and head control unit 54. However, the signal fromthe tracking loop can be generated by other conventional means.

In practice, variability in the gain of optical sensor 60 due to aging,temperature, humidity and lot to lot variations could result invariability of damping for the system. Furthermore, excessive dampingcould cause a sluggish and slow system while insufficient damping mayresult in too much overshoot during transients. Therefore it isdesirable to have the ability to control the extent of damping toaccount for the particular characteristics of a system. In other words,the damping is tunable.

Moreover, the ability to control damping is desirable in order to obtaina predictable response of the system in both the gain and phase of theloop. For example, for systems which we have modeled we have found thathigher damping provides more phase at the critical region, which isdesirable for phase margin and stability. However, lower dampingprovides more suppression in the critical region. Therefore there is anoptimum damping value that will satisfy both the phase and suppressionrequirements. Consequently we provide a calibration algorithm thatadaptively tunes the damping to maintain the optimized value.

We have found that controllable damping can be accomplished bycalibrating the feedback loop 104 using the LED current. This is truebecause LED current is proportional to the damping applied by thefeedback loop 104.

To calibrate the feedback loop 104 a calibration system 120 shown inFIG. 8 is used. The calibration system 120 includes a damping tuningalgorithm 122 which can execute in a microprocessor such asmicroprocessor 38. The system also includes two digital to analogconverters (DAC's) 124 and 126 to receive digital signals computed bythe damping tuning algorithm 122. The first DAC 124 transmits analogsignals to the summer 101 and the second DAC 126 establishes setting forthe LED current to the optical sensor 60.

In operation, the damped VCM position signal controls the position ofthe head assembly 28. As represented by summer 130, lateral motion ofthe tape is added to lateral motion of the head assembly, and theresulting total motion is measured by conventional servo control system44 to generate PES 132.

The damping tuning algorithm 122 sends a set of digitized sine wavesignals to the VCM amplifier reference DAC 124 that controls the linelabeled “From Tracking Loop” in FIG. 7. The damping tuning algorithm 122collects PES data as the tape is moved across the head at a constantspeed. The damping tuning algorithm 122 implements this as a backgroundtask as the tape is moving and the drive is not doing read/write datatransfer but rewinding or searching for a location. Therefore the normaloperation of the drive is not interrupted. This background calibrationcontinuously maintains optimum damping.

The damping tuning algorithm 122 uses a curve fit algorithm to calculatethe transfer function of PES to the sensor current LED setting using asecond order function fit. It does this for three different LEDcurrents, high, medium and low. Once the damping tuning algorithm 122has computed the damping coefficients for all 3 different LED currents,the algorithm 122 fits a third order function to calculate the LEDcurrent for the targeted damping coefficient. It can now load this valuein the LED current control DAC 126.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art having thebenefit of this disclosure that many more modifications than mentionedabove are possible without departing from the inventive concepts herein.The invention, therefore, is not to be restricted except in the spiritof the appended claims.

1. A method for damping the motion of the head of a tape drivecomprising: measuring the position of the head over time with a positionsensor; damping the motion of the head based on the measured position ofthe head over time; tuning the damping by varying a parameter of theposition sensor.
 2. A method according to claim 1 wherein the step oftuning comprises filtering the measured position of the head.
 3. Amethod according to claim 2 wherein the step of filtering the measuredposition of the head comprises applying a lead-lag filter.
 4. A methodaccording to claim 1 wherein the step of measuring the position of thehead over time comprises using an optical position sensor.
 5. A methodaccording to claim 1 further comprising calibrating the damping.
 6. Amethod according to claim 5 wherein said step of calibrating comprisesapplying a signal to the head and measuring the response of the head tothe signal.
 7. A method according to claim 6 wherein the step ofmeasuring the response of the head is accomplished by using a positionerror signal of the tape.
 8. A system for damping the motion of the headof a tape drive comprising: a position sensor for measuring the positionof the head over time; feedback means connected to the position sensorfor damping the motion of the head based on the measured position of thehead over time; tuning means for tuning the damping by varying aparameter of the position sensor.
 9. A system according to claim 8wherein said position sensor is an optical position sensor.
 10. A systemaccording to claim 9 wherein said optical position sensor comprises alight emitting diode and said parameter is the current applied to thelight emitting diode.
 11. A system according to claim 7 furthercomprising a damping tuning means for applying a signal to the head andmeasuring the response of the head to the signal.